Although the underlying mechanisms of fatigue have been studied in several disease conditions, the data are not conclusive, and this symptom remains poorly managed at present. Longitudinal studies related to treatment-related fatigue in prostate cancer patients have been conducted, but there are limited studies exploring the changes in physiologic mechanisms before and after radiotherapy which can identify individuals who are at risk for enduring fatigue during and after therapy. Fatigue is conceptualized as a multidimensional symptom which incorporates temporal, sensory, cognitive/mental, affective/emotional, behavioral, and physiological dimensions. This prospective, observational study is the first study to explore the molecular-genetic mechanisms underlying fatigue and the cluster of symptoms (urinary, sexual, gastrointestinal, psychiatric) experienced by prostate cancer patients undergoing External Beam Radiation Therapy (EBRT). The primary objective of the study is to describe the changes in the self-reported fatigue; urinary, sexual, and gastrointestinal symptoms; depression; fatigue catastrophizing; and health-related quality of life (HRQOL) experienced by patients with and without prostate surgery before, during, and after EBRT completion. The secondary objectives of this study are to investigate the pro-inflammatory cytokine profile of EBRT-related fatigue (TNF, IGF-I, IL-6, IL-8, TGF and ) and to determine changes in white blood cell gene expression using buffy coat collection before, during, and after EBRT completion and to relate these changes in the levels of these biological markers to self-reported fatigue; urinary, sexual, and gastrointestinal symptoms; depression; fatigue catastrophizing; and HRQOL scores. This study also aims to measure the skeletal muscle strength, activity levels and energy expenditure of patients before, during, and at completion of EBRT and relate these findings with self-reported fatigue, physical symptoms (e.g. urinary, gastrointestinal, and sexual functioning), depression, and HRQOL scores.